This application is based upon and claims the benefit of Japanese Patent Applications No. 11-294471 filed on Oct. 15, 1999, and No. 11-294468 filed on Oct. 15, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method and an apparatus for fabricating a high quality single crystal, which are especially suitable to be used for fabricating a high quality silicon carbide single crystal on a seed crystal attached to a seed crystal attachment member.
2. Description of the Related Art
A silicon carbide single crystal is expected as a semiconductor substrate for a power device, due to its high withstand voltage and high electron mobility. Generally, a single crystal growth method, so called a sublimation method (modified Lely method), is used for growth of a silicon carbide single crystal.
In the modified Lely method, a silicon carbide material is introduced into a graphite crucible, and a seed crystal is attached to an inner wall of the graphite crucible to face the material part. The material part is then heated up to a temperature of 2200xc2x0 C. to 2400xc2x0 C. to generate sublimation gas. Accordingly, a silicon carbide single crystal is grown, by recrystallization of the material, on the seed crystal having a temperature that is set to be lower than that of the material part by several dozens to several hundreds degrees.
In this conventional method, however, it is required to further improve crystallinity of the silicon carbide single crystal. For example as shown in FIG. 5A, the seed crystal 5 is attached to the inner wall of the graphite crucible 101 directly or through a protruding portion provided on a lid member 112 of the graphite crucible 101. Because of this, as shown in FIG. 5B, a poly crystal 8 is grown on the inner wall of the graphite crucible 101 exposed to the crystal growth ambient as the silicon carbide single crystal 107 is grown, and the poly crystal 8 is attached and fused to a peripheral portion 107a of the silicon carbide single crystal 107. Consequently, the peripheral portion 107a is poly-crystallized, and defects are produced in the silicon carbide single crystal 107.
To solve this problem, JP-A-6-48898 proposes to perform a step for stopping single crystal growth before the peripheral poly crystal becomes larger than the single crystal and restarting the single crystal growth after the peripheral poly crystal is removed from the graphite crucible, and repeat the step several times. However, because this step must be repeated several times for forming one single crystal ingot, the process is complicated. Further, when the silicon carbide single crystal should be grown at a high temperature of 2000xc2x0 C. or more, raising and lowering the temperature requires long time, and the process time for fabricating single crystal is significantly increased. Furthermore, it is very difficult to find out the timing of the peripheral poly crystal not exceeding the single crystal in size, i.e., the timing immediately before the poly crystal is fused to the single crystal, and to repeat it stably.
The present invention has been made in view of the above problems. An object of the present invention is to provide a method and an apparatus for fabricating a single crystal with high quality.
According to one aspect of the present invention, a single crystal is grown on a seed crystal disposed in a vessel. The vessel defines therein a growth space, and has a seed crystal attachment portion and a peripheral portion surrounding the seed crystal attachment portion. The seed crystal attachment portion has a support surface that is exposed to the growth space, and recessed from a peripheral surface of the peripheral portion. The seed crystal is attached to the support surface to cover an entire area of the support surface. In this method and apparatus, since the seed crystal attachment portion is not exposed to the growth surface when the single crystal is grown on the seed crystal, no poly crystal is grown on the surface of the seed crystal attachment member, and fusing of poly crystal to the single crystal can be prevented. As a result, the single crystal can be fabricated with high quality.
According to another aspect of the present invention, within a vessel such as a crucible, a single crystal is grown on a growth surface of the seed crystal, and a poly crystal is grown on a peripheral surface surrounding the growth surface to have a height approximately equal to a height of the single crystal. That is, the single crystal is grown to be embedded in the poly crystal. Accordingly, temperature distribution on the growth surface of the single crystal becomes uniform. In consequence, the growth surface of the single crystal can be kept flat even when the length (height) dimension of the single crystal is increased and no crack is generated in the single crystal.